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Protecting AC Output Ssrs Against Voltage Transient Phenomena

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Protecting AC Output Ssrs Against Voltage Transient Phenomena January 2012 Protecting AC Output SSRs valves, etc.), distribution equipment against voltage transient placed on the electrical supply (micro phenomena power cut, etc.). These phenomena exist on all industrial electrical supplies, and their incidence can in many cases be reduced by simply being aware of them and taking elementary precautions. A vital basic safeguard against all these phenomena is to select the correct size of SSR in relation to its application in order to take advantage of all the SSR characteristics with an adequate safety margin. SSRs are extremely reliable when Transient phenomena on input operated within their specified parameters. For those cases where The main problems encountered on an voltage or current transients may exist in SSR input come from conducted voltage the field that might otherwise exceed the surges. If such a voltage surge exceeds SSR's ratings, there are several the minimum turn-on voltage (1 to 3 different means available to mitigate volts) while the relay is in the offstate, those situations. In this statement we’ll the SSR output circuit may close for a deal with the voltage transients affecting period of time or until the output current AC output SSRs. First of all it is next passes to zero. If the SSR is in the important to understand the origins of on state, the output may open if the these transient phenomena: transient polarity opposes the control voltage and exceeds the power supply - an electrical origin conducted by the rating. A voltage surge that is too high power supply wires. As the I/O in either polarity could destroy the capacitive coupling is extremely low, optocoupler if this is not adequately these transients mainly affect the protected. power output circuit and not coupled to the input circuit. Input protection method - an electro-magnetic radiated origin whose interference mainly affects the The input of an SSR can be protected low-voltage structure of the SSR, such against possible voltage surges by as the input circuit comprising the adding a Zener diode, a TVS (bi- optocoupler. directional in case of AC input, mono- directional in case of DC input) or an RC However, whatever the phenomenon supply mounted in parallel on the input. may be, it is often caused by lightning, These components delay the relay switching on inductive loads (motors, Crydom Inc. 2320 Paseo de las Americas, Suite 201 San Diego, CA 92154 Tel.: +1 (877) 502 5500 - Fax: +1 (619) 210 1590 - E-mail: [email protected] www.crydom.com switching for several micro-seconds Transient phenomena on output (which will not have serious consequences, particularly with AC) and If, following a voltage surge, the voltage reduces the effects of a conducted or at the output terminals of an AC SSR radiated stray impulse. Furthermore, a exceeds the maximum permissible synchronous (or zero-crossing turn-on) direct voltage or breakdown voltage, the relay is "naturally" protected against the SCR or the TRIAC will switch on until effects of a stray impulse on the input as the current next passes to zero. long as the stray impulse occurs outside Because of the high switching speeds of the valid switching window. SCRs or TRIACs, switching can be The following is an example of a noise triggered by a very short pulse of only a absorption circuit with capacitor C and few μs. resistor R connected to an SSR. The value of R and C must be decided carefully. The value of R must not be too large or the supply voltage will not be able to satisfy the required input voltage value. The larger the value of C is, the longer the release time will be, due to the time required for C to discharge electricity. Typical values of RC network contain Increase in dv/dt direct voltage capacitors in the range of 0.010 to 0.1 microfarad and resistors in the range of This characteristic is linked to the 10 ohms to 1000 ohms. physical structure of the output element and in particular to the coupling Anyway, on the practical side, protection capacities between anode and cathode method should be more of concern for in an SCR or TRIAC. If the variation of AC input SSR which are connected to the voltage at the relay terminals is too the AC line. Usually, there are never rapid, this can result in an uncontrolled problems associated to DC input SSR turn-on. The severity of the as they are fed by a power supply. So, consequences of such a stray trip will the quality of the power supply being obviously depend on the application, but free of transients is important. these can, in certain specific circumstances, lead indirectly to the solid state relay being destroyed. Crydom Inc. 2320 Paseo de las Americas, Suite 201 San Diego, CA 92154 Tel.: +1 (877) 502 5500 - Fax: +1 (619) 210 1590 - E-mail: [email protected] www.crydom.com There are 2 different type of dv/dt : static current in the C capacitor, causing a and commutating. voltage drop in the load such that: Static dv/dt is also known as turn on Vd = V + (L* di/dt) - U dv/dt because a static dv/dt failure is when the relay should remain off, but goes into conduction as a result of a sudden change in voltage (e.g.: initial application of line voltage). Static dv/dt is associated with all types of loads/applications. Commutating dv/dt or turn off dv/dt occurs when the SSR is expected to stop conducting load current but continues to carry load current as a This voltage drop could protect the result of a high rate of rise of voltage at output SCR. The main disadvantage of the moment of turn off due to a large this type of filter is the significant voltage/current phase shift associated increase in the SSR off state leakage with reactive loads. Commutating dv/dt current. The leakage current of a failures are exclusively associated with snubber may be x10 or x100 the value turn off of inductive loads where there is of leakage current of the SSR itself a significant phase shift due to the (SSR is typically <0.1 mA while a power factor of the load. While snubber alone can be >5 mA). Some conducting, the SSR has a voltage SSRs have “snubber” filters, which across the semiconductor equal to the improve their performance. However, forward voltage drop, typically 1.2 volts, some SSRs use special SCRs which and then must rise to value of the line accept significant dv/dt variations. These voltage when turning off. If there is a relays are known as “Snubberless”. significant phase shift, the voltage may The value of R is selected to protect the be as high as peak line voltage. capacitor from too much inrush current and the capacitor has to be rated at something more than AC line voltage. Output protection method RC snubber networks address both 1 - RC network - "Snubber" static and commutating dv/dt. Snubbers address rate of change of voltage, while Mounting an RC element in parallel on TVS and MOVs (see next chapters) the output of the SSR both reduces the address amplitude. Obviously by dv/dt gradient generated by a stray changing the rate of rise of voltage, impulse and reduces the amplitude of snubbers may also effect amplitude this impulse by filtering, as long as the depending upon the source impedance impulses do not recur. of the supply. Any variation in the voltage at the terminals of an RC element results in a Typical value of a ”Snubber”: Crydom Inc. 2320 Paseo de las Americas, Suite 201 San Diego, CA 92154 Tel.: +1 (877) 502 5500 - Fax: +1 (619) 210 1590 - E-mail: [email protected] www.crydom.com Capacity : 0.022 uF < C < 0.047 uF 3 - Varistors (MOV) Some of Crydom Panel Mount SSRs To protect the SSR against high energy have the snubber already included in stray pulses, it is also possible to use a the relay or they have it as an option Metal Oxide Varistor (MOV). (S option). 2 - Bi-directional Transzorb (TVS) A “Snubber” filter alone is not usually The drawback with these is that they sufficient to protect an SSR effectively, lose their characteristics over time as a particularly against high-energy stray result of the stray pulses received. They pulses. Using a TVS (also called must be replaced after each incident. “Transzorb” or “Transil”) improves this The characteristic of a varistor is that protection reliably. Surge-suppression with a voltage at its terminals less than diodes are intended to protect electrical its nominal value, the impedance of the equipment which is sensitive to fast MOV is very great (several Mohms). transients of low or medium energy However, once that value is exceeded levels. They are designed to have very the impedance very quickly drops to less good performance where the needs of than 1 Ohm, with the response time for this type of equipment are concerned, the MOV being approximately 20 to 50 i.e. for overloads lasting close to one ns. millisecond or less. This type of component can also provide good Although the bi-directional TVS diodes protection against electrostatic provide very good performance for fast discharges (ESD). transients, the use of the MOV is a must in the case of a Solid state reversing With an alternating current a bi- contactor. Relays with internal TVS must directional version, or two “Transils” not be used in motor reversing mounted head to tail, should be used. applications. An internal TVS may switch on the output of the SSR when Most of the Crydom Panel Mount and subjected to an electrical transient, DIN Rail Mount SSRs have this option effectively creating a phase-to-phase available (P option).
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